SUMMARY: Polycystic kidney disease (PKD) is characterized by the development and persistent expansion of renal cysts. Progressive renal fibrosis is a classic feature of PKD, that accompanies cyst expansion and is often a final common pathway to end stage renal disease. Fibrosis is associated with chronic inflammation, myofibroblast activation and excessive extracellular matrix (ECM) remodeling. Currently, the mechanism for development of renal fibrosis in PKD is unclear and strategies for inhibiting fibrosis are lacking. The goal of this project is to determine the role of cystic epithelial cells in the production of pro-fibrotic factors and tubulo-interstitial crosstalk, stimulating myofibroblast activation and excessive ECM remodeling in PKD. In preliminary studies we found that a pro-fibrotic matricellular protein called connective tissue growth factor (CTGF) is significantly upregulated and secreted by cyst lining epithelial cells, and that CTGF production could be stimulated by vasopressin type-2 receptors (V2R) and yes-associated protein (YAP). Moreover, CTGF from PKD renal epithelial cells induced activation and migration of myofibroblasts, the primary producers of ECM. RNA-seq analysis suggested that CTGF-induced myofibroblast activation could be dependent on glycogen synthase kinase-3? (GSK3?) and early growth response-1 (Egr1). Hence, we hypothesize that, V2R stimulates YAP mediated CTGF production in PKD cystic epithelial cells, that leads to activation of myofibroblasts by a GSK3? and Egr1 dependent mechanism. Myofibroblasts continuously secrete large quantities of ECM which in turn can induce cyst expansion. The proposed studies will; (A) Determine the role of CTGF in the development of renal fibrosis in PKD, by testing the effect of anti-CTGF antibody on interstitial myofibroblast activation and ECM production and the effect of myofibroblast depletion on renal fibrosis and cyst expansion in mouse models of PKD. (B) Determine if CTGF regulates myofibroblast activation by a GSK3?, CREB and Egr1 dependent mechanism, and if fibroblast specific GSK3? gene deletion will reduce fibrosis and cyst expansion in PKD. (C) Determine how V2R signaling stimulates CTGF production by cystic epithelial cells, and test the effect of YAP inhibitor on interstitial fibrosis and cyst expansion in PKD. These studies are expected to demonstrate that the cystic epithelium plays an important role in modifying its microenvironment by regulating ECM producing cells, which in turn could promote progressive cyst expansion in PKD. Thus targeting key pro-fibrotic cell signaling targets in the cystic epithelium and interstitium could be a novel method to effectively stop PKD progression.